1. Field of the Invention
The present invention is generally related to broadband communications systems. More particularly, the present invention is related to broadband fixed wireless systems that use Data Over Cable Service Interface Specification (DOCSIS) or any of its derivatives.
2. Background Art
In DOCSIS related broadband communications architectures, data is transferred between a central location and many remote subscribers. The central location may be referred to as a headend for cable systems, a wireless access termination system (WATS) for broadband terrestrial fixed wireless systems, or a satellite gateway for two-way satellite systems. Subscriber equipment may be referred to as a cable modem (CM) for cable systems, a wireless modem (WM) for broadband terrestrial fixed wireless systems, or a satellite modem (SM) for two-way satellite systems.
In a broadband terrestrial fixed wireless system, the communication path from the WATS to the WM is called the downstream and the communication path from the WM to the WATS is called the upstream. Downstream processing in current broadband terrestrial fixed wireless systems transmit data using time division multiplexed (TDM) signals over a single channel with a fixed modulation type and forward error correction (FEC) coding rate. Such signals have a fixed spectral efficiency in bits per second/Hertz (bps/hertz). The spectral efficiency that can be achieved depends on signal to noise ratio (SNR) and channel characteristics, such as distortion, fading, group delay variation, etc. Signal parameters such as modulation type, FEC coding type, and FEC coding rate determine the SNR required for the WM to have error-free or quasi error-free operation in a given channel. In a typical deployment, WMs experience a wide range of SNRs and channel conditions. This results in a large range of potential spectral efficiencies.
There is a trade-off between receiver parameters that allow for high throughput (high order modulation and high FEC code rates) and those that allow the signal to be reliably received at low SNRs, but with a lower throughput (low order modulations and robust low FEC code rates). Bandwidth efficiency can be controlled by the WATS through the selection of the modulation order, such as QPSK, 16 QAM, 64 QAM, etc., and the type and rate of the FEC used. The lower the bandwidth efficiency, the smaller the data throughput on a given downstream channel. Lower bandwidth efficiencies imply the ability to operate at reduced SNRs and/or in degraded channels.
In real world environments, subscribers experience a wide range of path losses and channel degradations. For example, in the case where a WATS is broadcasting to WMs that are located over a wide geographic area, various degradations, such as partial obstructions, antenna misalignments, etc., cause the signal power levels and SNRs received by individual subscribers to vary significantly. For current DOCSIS based systems, where modulation order and FEC parameters are fixed for a given channel, the modulation order and FEC parameters must be selected to allow the worst case WM to operate reliably. In other words, the system must operate with parameters that allow the worst case subscriber to obtain service with a given probability of success. Thus, subscribers that could otherwise receive data at a higher rate are penalized by the presence of disadvantaged subscribers.
Thus, what is needed is a system and method of dynamically assigning data traffic with different modulation orders and FEC parameters to different WMs within the same downstream channel, referred to hereinafter as “downstream adaptive modulation (DS-AM).”What is also needed is a system and method that implements DS-AM in a manner that enables non DS-AM enabled WMs to efficiently continue operation.